- Machine Shop & Tool Repair Service, Plainville
- Machinery, Supplies & Equipment - Business Production Related, Plainville
- Machinery, Supplies & Equipment - Construction & Contractor Related, Plainville
- Magnetos, Plainville
- Mail Delivery Service, Plainville
- Mailing, Delivery & Courier Service, Plainville
- Major Appliances, Plainville - Household & Kitchen
- Management Consultants & Services, Plainville
- Manicurists & Pedicurists, Plainville
- Manufacturing, Plainville
- Masonry Materials & Supplies, Plainville
- Medical & Dental Management, Plainville
- Medical Billing Services, Plainville
- Medical Groups & Clinics, Plainville - Medical & Health Centers
- Medical Management & Business Administration Service, Plainville
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- Membership Sports & Recreation Clubs, Plainville
- Metal Buildings, Plainville - Kits, Homes, Steel Buildings, etc
- Methodist Church, Plainville
- Mexican Restaurants, Plainville
- Millwrights, Plainville
- Mining, Plainville
- Miscellaneous Government, Plainville
- Mobile Homes, Plainville
- Mobile Homes Manufactures, Plainville
- Mortgage Bankers & Correspondents, Plainville
- Motor Vehicle, Plainville
- Motor Vehicle Rental & Leasing, Plainville
- Moving Supplies & Equipment Rental, Plainville
- Moving Truck Rental , Plainville
- Muffler Shop & Exhaust Systems, Plainville
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